This invention relates to transmission systems using multicarrier modulation, and is particularly concerned with frame synchronization in such systems, referred to below for brevity simply as multicarrier systems.
The principles of multicarrier modulation are described for example in xe2x80x9cMulticarrier Modulation For Data Transmission: An Idea Whose Time Has Comexe2x80x9d by John A. C. Bingham, IEEE Communications Magazine, Vol. 28, No. 5, pages 5-14, May 1990. As is known, in a transmission system using multicarrier modulation, FDM (frequency division multiplexed) sub-carriers spaced within a usable frequency band of a transmission channel, forming a set of sub-carriers, are modulated at a block or symbol transmission rate of the system. The bits of input data for transmission within each block or symbol period are allocated to the sub-carriers in a manner which is dependent upon the signal-to-noise ratios (SNRs) of the sub-carriers, typically so that the bit error rates of the sub-carriers, as monitored at the receiver, are substantially equal. As a result, the different sub-carriers carry different numbers of bits in each symbol period. With an appropriate allocation of bits and transmit powers to the sub-carriers, such a system provides a desirable performance.
One particular form of multicarrier modulation, in which the modulation is effected using a discrete Fourier transform, is referred to as discrete multitone, or DMT, modulation. The related applications referred to above disclose details of multicarrier systems using DMT modulation.
As in any communication system, it is necessary to establish and maintain synchronization between the transmitter and receiver of a DMT or other multicarrier system. Frequency synchronization is conveniently provided in a DMT system by using one of the multiple tones as a pilot tone to control a phase locked loop at the receiver, as indicated in Standards Committee Contribution T1E1.4/93-022 by J. S. Chow et al. entitled xe2x80x9cDMT Initialization: Parameters Needed For Specification In A Standardxe2x80x9d, Mar. 8, 1993. This reference also outlines other initialization processes of a DMT system, including the allocation of bits to sub-carriers or tones of the system.
In addition to this frequency synchronization, synchronization of the transmitted blocks or symbols of data is required. This is referred to herein as frame synchronization, each frame corresponding to one block or symbol of the multicarrier system, for consistency with the same term as used in single carrier transmission systems. It should be appreciated that each frame, block, or symbol can comprise a substantial amount of information, for example about 1700 bits (providing a transmission rate of about 6:8 Mb/s with a symbol period of about 250 xcexcs).
A single carrier transmission system, for example a QAM (quadrature amplitude modulation) system, usually operates entirely in the time domain. In such a system, a relatively xe2x80x9crandomxe2x80x9d frame synchronization sequence can be used to maintain frame synchronization, the sequence being inserted directly into the time-domain signal sample stream at the transmitter and being extracted and correlated with a stored copy of the sequence at the receiver. A large correlation result indicates that frame synchronization has been maintained, and a small correlation result indicates a loss of frame synchronization, i.e. that there has been a slip by an unknown number of time-domain samples. In the latter case the receiver instigates a search procedure to resynchronize the receiver, i.e. to re-align the frame boundaries at the receiver to those at the transmitter.
This time domain frame synchronization provides a simple yes or no answer to the question of whether the receiver is frame synchronized. To resynchronize the receiver when frame synchronization is lost, the system may be required to correlate and search through a large number of possible frame alignments. This is a time-consuming, and hence undesirable, procedure.
An object of this invention is to provide an improved method of providing frame synchronization in a transmission system using multicarrier modulation, and an improved transmission system which makes use of this method.
One aspect of this invention provides a method of maintaining frame synchronization in a multicarrier modulation transmission system in which a synchronizing frame containing a synchronizing pattern is periodically transmitted, comprising the steps of: storing complex amplitudes of the synchronizing frame; correlating the complex amplitudes of the synchronizing frame with stored information representing the synchronizing pattern, thereby to produce a correlation result; and determining whether the correlation result falls below a threshold value, indicating a loss of frame synchronization, and in this event: performing a plurality of correlations between the stored information and the stored complex amplitudes in each case multiplied by a respective complex value representing a respective complex derotation of the stored complex amplitudes, each complex derotation corresponding to a respective time shift of the synchronizing frame, thereby to produce a plurality of correlation results each corresponding to a respective time shift; determining from the plurality of correlation results a time shift for restoring frame synchronization; and adjusting a frame boundary in accordance with the determined time shift to restore frame synchronization.
For a discrete multitone modulation transmission system, the method preferably includes the steps of: using a tone having a predetermined frequency for frequency synchronization between a transmitter and a receiver of the system; at the transmitter, converting complex amplitudes in the frequency domain into time domain values using an N-point Inverse Fast Fourier Transform, sampling time domain values at the transmitter at a sampling frequency which is j times the predetermined frequency, where j is an integral power of two; and at the receiver, converting time domain values into complex amplitudes in the frequency domain using an N-point Fast Fourier Transform; each of said complex derotations corresponding to a respective one of N/j time shifts within the duration of one frame. This is particularly advantageous if the synchronizing frame is periodically transmitted once every Q frames, where Q is an integer greater than N/j, because it enables frame synchronization to be restored between two consecutive synchronizing frames.
Preferably each correlation result is produced by multiplying each complex amplitude by a corresponding complex amplitude from the stored information representing the synchronizing pattern, and summing the real parts of the complex products. The method preferably includes the step of weighting the complex amplitudes being multiplied, the weighting for each complex amplitude being multiplied preferably being dependent upon a signal-to-noise ratio of a multicarrier channel associated with the respective complex amplitude.
Another aspect of this invention provides a multicarrier modulation transmission system receiver comprising: a Fast Fourier Transform (FFT) unit for transforming time domain values into complex amplitudes in the frequency domain; a buffer for supplying received time domain values to the FFT unit in accordance with a frame boundary; a correlator for correlating complex amplitudes of a synchronizing frame of the system with a synchronizing pattern stored at the receiver to produce a correlation result; and a control unit responsive to the correlation result being below a threshold value to adjust the frame boundary by a time shift determined by performing a plurality of correlations between the stored synchronizing pattern and the complex amplitudes in each case multiplied by a respective complex value representing a respective complex derotation of the complex amplitudes corresponding to a respective time shift of the synchronizing frame, and selecting the best correlation result.